At the present time, the availability of means for ensuring the protection of a sensitive zone against possible intrusions constitutes an issue that is both significant and poorly mastered. Indeed, depending on the size of the zone to be protected, the nature of the terrain constituting this zone, the permanent or temporary character of the protection deployed as well as the degree of selectivity sought as regards objects that ought to be considered intruders, the solution to be implemented can take substantially different forms.
Based on the type of protection desired and criteria defining this protection, it is therefore necessary to determine the most appropriate type of protection systems from among those available on the market. However, at the present time, the proposed solutions are very few and do not generally make it possible to simultaneously satisfy all the criteria defining the desired type of zone protection.
A known protection scheme consists in installing a perimeter monitoring system comprising sensors disposed more or less regularly, on the ground or at a given height, along a line delimiting the entrance or the outline of the sensitive zone, these sensors having the main role of detecting the crossing of this line by one or more intruders. In a known manner, it is for example possible to use, depending on the case, simple seismic sensors or in a more sophisticated manner barriers of infrared sensors.
The detection elements used for this type of protection are generally defined by taking account of the temporary or permanent character of the barrier deployed. They are also defined as a function of logistical criteria such as, for example, the ease of transporting these elements on a man's back or on a lightweight vehicle or else their swiftness of implementation to form the sought-after protection barrier.
Systems such as those mentioned have an advantageously simple manner of operation. Moreover they allow effective detection of an intruder whatever the environment in which they are disposed. They can thus be installed in a wooded zone. On the other hand, they exhibit a certain number of drawbacks:                the protection that they offer is very generally limited to the signalling of an intrusion, at the actual instant at which the barrier thus constituted is crossed. In the most favourable case this signalling is supplemented with a more or less precise item of information, relating to the spot where the barrier has been crossed. They do not therefore make it possible to implement an early signalling procedure, making it possible to be informed of the approaching of the zone to be protected by possible intruders.        the item of information provided is limited to the noting of the crossing of the detection barrier by an intruder. After detecting an intrusion, these systems do not generally provide any further information relating to the progress of the intruder in the protected zone so that in the absence of a very fast reaction, the search for the intruder or intruders entails a complete exploration of the protected zone.        
Moreover, in addition to the drawbacks cited above, systems using optical or infrared detectors generally exhibit the drawback of becoming inoperative when the atmosphere becomes too humid.
Another known protection scheme consists in installing a monitoring system based on the implementation of a short-range surveillance radar operating for example in the S band or in the X band. The use of such a system exhibits numerous advantages:                It makes it possible to carry out at one and the same time the detection of the crossing of the line defining the entrance to the zone to be protected and the monitoring of the space constituting this zone. Under certain utilization conditions, the use of a radar system also makes it possible to carry out detection of approach to the protected zone.        It makes it possible to produce a detection system which is very intensitive to climatic conditions.        It also makes it possible, in so far as the detection means implemented are radar means, to ascertain the speed of progress of a detected intruder. It is thus advantageously possible to determine the category, animal, pedestrian or vehicle for example.        Finally it makes it possible to carry out the detection of objects moving on the ground at a relatively slow pace, and also the detection of objects, generally faster, flying at low altitude.        
Hence, it is possible to determine at one and the same time the nature of the intruder, the position of the intrusion point, as well as the movement of the intruder inside the sensitive zone.
Moreover, the use of a barrier consisting of such a radar also advantageously makes it possible to simplify the conditions of utilization of the system for monitoring the zone, by mobilizing only a single operator.
On the other hand, the deployment of such radar equipment exhibits drawbacks too:                It is generally expensive, the radar equipment used generally being sophisticated;        It is limited in its conditions of use, in so far as the use of a short-range monitoring radar operating in the S band or in the X band is inoperative as soon as the space covered exhibits elements that could obstruct the direct propagation of the waves emitted, clumps of trees for example.        
Thus, if the zone to be protected comprises wooded areas, the coverage of these areas is not ensured, so that an intruder can exploit this absence of coverage so as to penetrate into the zone and progress therein without being immediately detected.